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Gambusia (formerly known as mosquito fish)

Introduction

Learn why Gambusia affinis were originally introduced to New Zealand and how this pest fish is now negatively impacting our freshwater ecosystems.

Male gambusia

Gambusia (Gambusia affinis) are sometimes referred to as mosquito fish due to the mistaken yet unfortunately widespread belief that they can control mosquito populations.

The first shipment of gambusia from the Gulf of Mexico to New Zealand was released into an Auckland Botanical Gardens pond in the 1930s. Little information about successive releases is available, but further transfers into Northland, Taranaki and Wellington in the 1930s are documented.

Since then, gambusia have continued to increase their range in many North Island waterways due to natural spread and by further illegal introductions.

Description and life history

Female gambusia

Gambusia are small fish with a greenish silvery sheen. Mature females grow to 6 cm and males to 3.5 cm. They mature at six weeks old and are short lived but breed rapidly and repeatedly enabling populations to build up to large numbers very quickly. Females give birth to live young. Consequently only one pregnant female is needed to start a new population.

What damage do they do?

Gambusia are aggressive and frequently attack native fish, nipping at their eyes and fins - endangered galaxiids and mudfish are especially vulnerable.

Gambusia also competes with native fish for food and have been known to eat native fish eggs.

A single gambusia female produces several broods a year and around 50 offspring per brood, offspring which can reach sexual maturity in as little as three to four weeks. Given such prolific reproductive behaviour, gambusia can quickly expand to outnumber native species and take over a waterway once they are introduced.

Where are they found?

Female gambusia found in a farm pond

Gambusia live in the shallow margins of slow flowing ponds, wetlands and streams, particularly around aquatic plants. They can tolerate poor water quality, high salinity levels and a wide range of water temperatures.

Populations exist throughout Northland, Auckland, Waikato, Bay of Plenty and Hawkes Bay, with isolated populations found around Whanganui.

For mosquito control

Native fish such as whitebait, bullies and eels, and aquatic invertebrates all feed on mosquito larvae.

If you have a pond, make it as unfriendly to mosquitoes as possible by creating steep sides, and have flowing water, and planting plants around the edge to shade the water. This will improve your pond's habitat for other species that feed on mosquitoes.